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Topic: Simple generator (Read 30189 times)

When the rotor is vertical as illustrated most of the flux will prefer to go through the bridging stator piece due to a shorter flux path which has a lower permittivity. When the rotor spins 90° there will be no reason for the flux to still go through the bridging piece, and hence no flux would be going through it. Going from maximum flux to 0 flux will of course cause the coil to generate a voltage.

In the illustration a tiny air gap can be seen as well. I noticed in experiments and femm simulations that having the right permeability is key. Put simply the toroid must have a lower permeability than the bridging piece. If this is not the case the flux will mostly travel through the toroid and not the bridging piece.The permeability of the toroid can be artificially lowered by introducing an air gap. The air gap need not to go all the way through, a small cut will do however it needs to be symmetrical on both sides or you'll create an asymmetric setup which is not advantageous.

The back torque associated with most electrical generators can hardly be seen in this setup even the cogging due to the air gap is minimal. This has to do with the fact that the field interactions happen mostly transverse to the motion of direction.

Sadly I destroyed my silicon steel toroidal core which I was experimenting with when trying to saw a small airgap through it however the second one is on its way and thought to share the concept in the meantime.

As you rotate the rotor you will be generating eddy currents in the ring stator... this will cause a drag on the rotor, heating of the stator, and is of course a consequence of the changing flux in the stator as the magnets sweep past. Isn't this also a consequence of Lenz's Law?

Yes but either using laminations or ferrite ring the eddy current issue could be minimized. Of course machining either the air gap in the ferrite or forming the stator from laminations surely makes tinkerer's life harder...

Yes but either using laminations or ferrite ring the eddy current issue could be minimized. Of course machining either the air gap in the ferrite or forming the stator from laminations surely makes tinkerer's life harder...

Gyula

The laminations would have to be parallel to the slots, through the thickness of the material, not the flat way parallel to the ring itself, to reduce eddys much, I think. A nonconductive ferrite might work, I don't know if they heat up when subjected to a changing field or not. The ferrite core pieces from a CRT flyback transformer, which is split, with tiny thin spacers in the split, might work for a trial even though it isn't exactly a "ring" shape.

In the first picture you can see how I ignorantly started cutting the laminated toroid from the outside to the inside. This caused the toroid to open up with great force as it had nothing to hold it in.

In the second attempt I started cutting the toroid radially, the material is holding its own now, that is until you cut all the way through. The hot glue trick in the second one was also a lesson learned quickly. Since the core is not glued together, as you cut radially you start loosening the inner/outer windings of the toroid. This starts out very Innocently but becomes a royal pain in the ass as you go. So either use epoxy or hot glue to hold the laminates, near the cut, together is a smart move.

Ideally I would hit alibaba.com and ask some Chinese manufacturer to send me a perfectly cut toroid .

I also have some ferrite toroids, this seem to show the effect of low vs high permeability quite well. They don't need an air gap as the bridging piece is attracted quite well to the toroid. However I have no clue about the properties of these specific toroids, but I would guess that the permeability is too low and they also seem to have an awfully bad retentivity, they are permanently magnetized in some parts just by having had neo mags close to them at some point in time.

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I would also like to add that there are many different takes on this concept I just picked the one that I thought was most practical to build.

Below are some variants. The first uses motor laminates the kind made for slotted motors. Here the rotors is on the inside to prevent eddy currents.

The second is a bit more special in the sense it has one air gap and uses the toroid itself as the generator. So the coil is wound around said toroid. Again since there's a higher permittivityon one side the flux will predominantly flow to one side untill the rotor hits the 90° mark and again you'd have a max to 0 flux scenario which would generate a voltage. However as far as I know the generated toroidal current should have no effect on the spinning rotor.

For those wishing to attempt a build these ideas, I list some toroidal core offers. Unfortunately the cores with reasonable OD for such setups are not really cheap. Most of the cores below have a know permeability.

Of course on ebay you can also find some (OD 2.25" and 2.9") toroids at seller 'alltronics' for instance or others.

I have found an interesting and unusual ferrite cutting "method" albeit it is shown for ferrite rods, it is rather a 'braking' method because it actually brakes the material. But I think the full air gap instead of the partial airgap may not make much difference in the operation of such setup. Nevertheless, it is a risky method for sure because of the cost of the larger OD toroids.This is the link to Clanzer's video on ferrite rod "cutting": http://www.youtube.com/watch?v=cUWn5m8ASC0

But in fact as broli wrote above, ferrite toroids may not need cutting at all in these setups.

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Perhaps you could ask the Magnetec people what or how they suggest the cutting should be done? Diamond or maybe laser cutters are preferred?? be very careful and maybe look for a good machine shop once you know the how to.

It has been a while since the birth of this concept, sadly nothing much has happened since.

However I'm trying to get the ball rolling again, or better yet, the generator spinning. I got rid of the idea that using a toroidal core might do the trick. The Achilles heel of this concept is unwanted airgaps, and when you coalesce multiple materials together you are playing with so many variables. Hence the new idea to stamp a custom stator core to reduce bridging materials. It makes the generator even more simple I believe as you only need one airgap to worry about and it's on the outside too.

I sort of reached the limitations of what simulation software has to offer. The results don't like to converge and become almost meaningless when reaching small airgap sizes. That and the simulation time increases exponentially with element size.However in general the torque does decrease the smaller the airgap gets. Femm being the most stable simulation software confirms this fact.

Currently I sent a few quote requests to some Chinese suppliers on alibaba for custom rotor laminates as attached per pdf, to ease the study of the above.Any other suggestions are welcome as it seems like Europe no longer does anything in regards to stamping motor cores.

While waiting on responses from the couple core stamping companies, one would like to charge a wopping 3500$ to create the mold alone, in china I was thinking of cheaper ways on building a generator out of the same principle. One way that came up is using cheap EI audio transformers that can be bought everywhere. These can then be arranged in such a way to form a circle. Certainly it has its own engineering challegens but this is a much cheaper alternative than stamping custom laminates.

You mentioned audio transformers as off the shelf components and this remembered me to common mode line filters or chokes used at the mains input of many switch mode power supplies. There are such chokes with two ferrite C cores back to back or two ferrite E cores also back to back, usually with a little air gap between the facing surfaces. See some here and these are available in most component shops, I chose Digikey, they have several mechanical sizes with several coil inductance values.

These chokes have double windings next to each other so you could connect them in parallel or in series, or use each separately in case of the C cores.

I am a bit surprised that the simulation shows most of the flux going through the leg that the magnet is just covering, I would have expected more flux going via the side legs because those side legs actually shunt the center leg. It is true though that the side legs each has half as much cross section area than the center leg does (but the two side legs alltogether have equal cross section to that of the center leg).

Maybe the C cores (that would have two legs only vs the 3 legs of the E cores) could also give useful induction, as per common sense, because then there would be two shunting flux pathes vs the three.

So I think these common mode ferrite chokes would serve well to build a cheap low power prototype from this setup, it would not need expensive lamination cuttings.